Wireless communications often rely upon the controlled radiation/reception of radio frequency (RF) signals. For example a wireless communication system may utilize an antenna array adapted to radiate and/or receive signals within predefined areas, such as through the use of directional antenna beams. Such antenna beams may be formed using an array of multiple antenna elements arranged in a particular pattern, such as spaced a certain wave length apart, designed to form desired antenna beam patterns based on the amplitude and/or phase of the RF signals that are provided to drive each element within the array.
In a phased array antenna system, a beam forming matrix, such as a Butler matrix, providing antenna beam interfaces for coupling with base station transceiver equipment through multiple paths and antenna element interfaces for providing RF beam component signals to the antenna array. Accordingly, the beam forming matrix includes circuitry for controlling the phase and/or amplitude of generated RF beam component signals. For the system to function predictably and/or to consistently form desired antenna beams, beam forming relies on a known distribution of phase and amplitude among the RF signals in getting to the RF feed points of the antenna array. That is, for the system to provide consistent and predictable beam forming, the beam forming matrix should provide the antenna array signals with acceptably certain phase and amplitude characteristics.
However, there is often an appreciable length of transmission line and a number of components, such as amplifiers and filters, disposed in the transmission lines between transceiver equipment and the antenna array. There are often differences in the physical and/or electrical delays of the multiple paths, e.g., individual antenna beam signal paths, coupling an antenna array to associated transceiver equipment. Accordingly, the phase of signals as provided to a beam forming matrix may experience differing amounts of delay and, thus, result in undesired phase differentials there between. Moreover, there may be various electrical and/or physical path differences within a beam forming matrix. Accordingly, the phase of signals as provided to the antenna elements of an antenna array may experience differing amounts of delay and, thus, result in undesired phase differentials there between. Phase differences on the order of a few nanoseconds when providing beam forming for signals in the 800 MHz frequency range and up can result in significant degradation of the desired radiation pattern.
Signals provided to a beam forming matrix and/or to the antenna elements of an antenna array may also experience different levels of amplification and/or attenuation. However, signal amplitude differentials are generally not experienced in sufficient magnitudes to significantly affect beam forming attributes.
Controlling (or knowing) the signals' relative phases after propagating through the different transmission paths is difficult because the different paths affect the phase of each signal differently. These difference are attributable to the transmission paths having different associated delays. The differing delays result from the paths having different electrical lengths and non-linearities associated with high order filters with steep roll-off characteristics.
Often systems are phase calibrated, such as at the time of assembly or deployment, to account for known transmission path delay differences, such as those associated with physical line length differences of multiple transmission paths. However, such calibration is generally only effective for a particular operating frequency. For example, RF signals as provided to a beam forming matrix, or as provided to the individual antenna elements of an antenna array, can be phase balanced at one frequency, but generally will not remain phase balanced within acceptable limits over a wide band of different frequencies.
However, communication systems, such as cellular telephony systems, often perform over a relatively wide frequency range (e.g., 40-50 MHz range of the 800 to 900 MHz spectrum of an advanced mobile phone service (AMPS) system). Accordingly, a system calibrated for a particular frequency of this frequency range may experience desired beam forming with respect to signals modulated at this carrier frequency while signals modulated at other frequencies of the frequency band may experience beam forming of a less desirable quality. Unfortunately, it is not practicable to re-calibrate a system for separate operating frequencies. This problem is compounded when the frequency range is further widened. For example, if a common transmission structure were to be utilized both for providing traditional analog AMPS cellular telephony communications and digital personal communication services (PCS) communications, thus requiring the system to operate over the frequencies of both services, the range of operating frequencies would vary even further from the calibration frequency.
Accordingly, what is needed is a method and system for providing to an antenna array signals having know phase relationships over multiple and different transmission paths across a desired frequency range.